Four-stroke engines are now being installed in personal watercraft to meet present and future stricter environmental and emission regulations.
To boost the power output of a four-stroke engine such that smaller displacement engine can be used, manufacturers of personal watercraft have, in some cases, equipped the four-stroke engines being used with a supercharger. A supercharger accomplishes this by forcing more air into the combustion chamber. More air means more fuel can be added into the combustion chamber, and more fuel means a more powerful explosion and greater horsepower.
A supercharger increases intake by compressing air above atmospheric pressure, without creating a vacuum. This forces more air into the engine, providing a “boost.” With the additional air in the boost, more fuel can be added to the charge, and the power and torque of the engine is increased.
A supercharger is mechanically driven by the engine's crankshaft either directly through gears or by belt- or chain-drive from the engine's crankshaft which wraps around a gear that rotates the compressor of the supercharger. The rotor of the compressor can come in various designs, but it always draws air in, squeezes the air into a smaller space and discharges it into the intake manifold thereby achieving forced air induction and higher power output for a given engine displacement.
To pressurize the air, a supercharger must spin more rapidly than the crankshaft of the engine driving it. The multiplication of rotation speed of the crankshaft is typically achieved through gear multiplication. To multiply the rotation of the crankshaft, the drive gear connected to the crankshaft is larger than the compressor gear of the supercharger thereby causing the compressor to spin faster than the crankshaft. Superchargers can spin at speeds as high as 60,000 rotations per minute (RPM) and the multiplication ratio between the crankshaft and the compressor gear is therefore in the range of 1:4 to 1:12.
A personal watercraft is generally quite sporting in nature and normally accommodates at least the rider on a type of seat on which the rider sits in a straddle fashion. The passenger's area is frequently open through the rear of the watercraft so as to facilitate entry and exit of the rider and passengers to the body of water in which the watercraft is operating. A personal watercraft is generally quite small compared to a boat, and due to its sporting nature, it is fast and agile and its mechanical components are subjected to pounding as the personal watercraft hits the water.
During operation, the propulsion system of the personal watercraft may become momentarily disengaged from the water thus causing thus subjecting the engine to large variations in engine load and torque. As well, the supercharger of the engine, which is mechanically powered by the crankshaft, is subjected to large variation in rotation speed and torque due to the engine's load variations. Furthermore, every combustion in each individual cylinder produce a torque peaks on the crankshaft which are transmitted to the supercharger. As the engine and supercharger speed and torque fluctuate continuously, the supercharger is less efficient than it would otherwise be in a more stable environment. Also, the various components of the supercharger are exposed to increasing mechanical loads which increase wear and reduce the durability of the supercharger.
To alleviate this problem, a friction clutch or a one-way clutch has been coupled directly to the gear rotating the compressor of the supercharger in order to reduce the variations in rotation speed of the compressor by slipping when there is a rapid change in engine torque and speed.
However, due to the gear multiplication between the crankshaft and the supercharger previously mentioned, a friction clutch or one-way clutch coupled directly to the compressor gear of the supercharger and therefore rotating at the same speed as the compressor can only reduce a small portion of the variations in rotation speed of the compressor. The high rotational speed of the supercharger shaft and therefore the high centrifugal forces exerted on the clutch limits the size of the clutch to a small diameter clutch. A small diameter clutch is subject to high specific heat input especially considering that the continuous torque peaks caused by every combustion in each individual cylinder. The small clutch performs microslips in every cycle of the engine and generates heat continuously which causes heat build-up and increase wear and reduce the durability of the clutch.
Thus, there is a need for a supercharged engine having a dampening system for the supercharger that reduce variations in rotation speed and torque of the supercharger due to engine torque variations.